I agree with the comments above. Arlo wanted to get at Sarah. He was jealous and wanted to do something that would cause her to react. She foiled his plan when she took responsibility for the cut pony tail. Yet, she did tell her mother that Arlo cut off her hair. It states in the book, “ He’d been waiting for punishment, but instead there was this: no rebuke, no drama, nothing at all.”
I think Arlo is like a feral cat. He wants to belong to something, he thinks he wants what he sees, a family, a home, stability, but too much time has passed and the attention, the structure is too much, too foreign he has to take steps to ruin it, to cause it to fail, and he can’t help it. I also found the scene disturbing. I think it put Arlo a bit in awe of Sarah’s power and self awareness. It changed how he viewed her going forward. Later, on pg 176 Sarah asks him why he cut off her hair. And he wants to know why she didn’t tell. Apparently, he never knew that she had. He saw her refusal to acknowledge what he’d done chilling “ as if he didn’t exist at all” and her composure “ haunted him”. Sarah feels “guilty for the life she had, as if in coming along she’d taken Arlo’s life away, as if life were a meal and she’d eaten more than her fair share of it.” I think the entire episode, understandably left them uneasy around each other and untrusting as well.